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RE: performance/tuning question
Hi John,
At 05:10 PM 7/27/00 -0700, you wrote:
>
>Terry -
>
>Many thanks for all that information on the NST "sweet spots". I had heard
>about these possible conditions but had little specific information. I would
>be interested in more information so these special conditions can be
>incorporated into a TC program. The resonant cap is no problem but how about
>those other "sweet spots". At present it looks to me like they are not
>changing Tesla coils outputs very much.
>
>For the resonant cap I use the following equation.
> C = 10^6 * I / (6.283 * Hz * V)
>
>For a 15/60 900 watt NST - 60 Hz
> C = 10^6 * .06 / (6.823 * 60 * 15000) = .0106 uf
>
>The gain in current or wattage is about 2 to 3 times normal.
>
>I agree it is difficult to compare spark outputs for these special
>conditions. However, with a TC program you show something approximate based
>on the available empirical data. Hopefully coilers who have coils will test
>them so more information is available.
>
>What is the LTR line frequency timing theory?
The best LTR explanation I have seen is/was Richie Burnett's web page at:
http://www.staff.ncl.ac.uk/r.e.burnett/
Unfortunately, I don't think it is still working?!?! My paper at:
http://users.better-dot-org/tfritz/site/papers/modact/modact.html
Describes my large LTR coil. During the optimization of this coil by trial
and error, I chased for higher and higher efficiency by changing all the
values. As it turned out I could get the best power out by using a big
primary cap (24nF) and very carefully timing the spark gap firing. The gap
actually fires significantly after the peak cap voltage, as odd as that sounds.
There is no book or anything that describes this well. We really need to
find out what happened to Richie's wonderful site! (and copy it this time :-))
The third case can be demonstrated (very risky to NST) by hooking a 30nF
cap to a large NST and turning up the variac slowly. There will be a
sudden point when the NST will "growl" very loudly and the current and
voltage will jump to extreme levels. Unfortunately, the NST will destruct
in a matter of seconds. I "play" near the edge with my small LTR coil, but
I am not sure there is an easy safe way to use the power available there.
I have coined the term "SLTR" for Saturating Larger Than Resonant to
describe coils that use this. I have only heard of my little LTR coil that
has this capability but I don't use it... I posted two old note at the end
here on this. The archives say much more an all this as does the following
text file of the last six months of posts if you have a program that can
search it for keywords.
http://users.better-dot-org/tfritz/1-6OF00.ZIP
Cheers,
Terry
>
>The people with pig powered systems do just the opposite. They do not want
>and can not handle the possible full output power of the transformer when it
>is short circuted by Tesla coil operation. They limit the potential current
>that can be over 50 times normal or 50 times KVA! Fortunately the limiting
>by the electric service, ballasts, etc, saves coilers from desaster. Coilers
>should be aware that this type of hazard can cause their electric service to
>explode as you pointed out.
>
>John Couture
>
>X-Sender: twftesla-at-pop.dnvr.uswest-dot-net
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Date: Sat, 15 Jan 2000 14:59:13 -0700
To: tesla-at-pupman-dot-com
Subject: New LTR behaviour and warning!!
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From: Tesla List <tesla-at-pupman-dot-com>
Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
Hi All,
I was working on my new little coil today. I have found a new phenomena
that needs to be considered in LTR design. I took the following readings
as I was turning up the variac. This is a new Actown 30mA 9000V neon into
a 23.8nF MMC primary cap.
voltage in Current out Vout1 Vout2 Zout Step up
10 2.5 200 199 199600 39.9
20 6 395 397 132000 39.6
30 9 586 590 130667 39.2
40 13 772 777 119153 38.7
50 16.5 967 972 117515 38.8
60 20.1 1175 1185 117413 39.3
70 24 1372 1386 114917 39.4
80 28 1600 1615 114821 40.2
90 35 1982 2013 114143 44.4
95 42 2495 2500 118928 52.57
You will notice that as the current approaches about 150% of the
transformers rating, the current and voltage start to ramp up very quickly
in relation to the supply voltage. Although I didn't take the time to
measure it well, I have had the output voltage up to 3500 volts where the
safety gaps (set way down) start to arc. I would guess the output current
was about 60mA at 7000 Volts out during that. That is about 3 times what
the transformer was designed to supply (420VA). The new Actown transformer
is "proving" to be very tough ;-)) The output voltage divided by the
current (Zout) seems to stay constant during all this. The transformer's
unloaded step up ratio is 75. It appears that the transformer is going
toward that ratio again, despite the capacitor load, as the current increases.
I think what is happening is the shunts in the transformer are saturating
and it is loosing it's ability to limit current. Although being able to
make the shunts "go away" is sort of neat, it can cause very high stress on
the windings in the form of high current heating. In my case I was
dissipating about 5.5 times the normal heat load in the windings (I didn't
keep it there very long ;-)) This could destroy the transformer through a
"new" mechanism. Unlike the resonant rise phenomena, the voltage does not
"skyrocket". Only the current capability of the transformer is increasing.
This leaves a possible failure mechanism in LTR coils that is not
protected against! The spark gaps will not save the day and typically we
do not have tight high-speed current limiting in the system. However,
carefully chosen fast blow fuses on the transformer's AC input could blow
and limit the current. I will probably now recommend fast blow fuses on
the AC transformer inputs of LTR coils to protect against this. This may
explain why some LTR coils seem to give longer sparks than others despite
all the reasons they should not be able too. This saturation may be
providing them with unexpected power!
This places another variable in LTR Tesla coil design. On one hand, we can
now pull just as much voltage and current we want out of a neon. On the
other hand, we can destroy them in a whole new way... I will have to go
back and redesign my coils charging circuit (again...) to account for this.
I also should add fast fuses to the transformer input too.
This may also provide a neat way to (destructively) test neons for their
"real" current ability. I suppose you could get two transformers. On one,
you could measure the over voltage breakdown point on one leg and the over
current blow out point on the other leg. Then use that data to design an
"optimal" coil to use the other transformer on. It would be fun to get a
number sample of transformers from a bunch of different manufacturers and
do the "consumer reports" comparison on how tough they are. The results
would be fascinating... I could see the sales brochure of the winner, a
big Tesla coil on the front with "we're #1" on it... ;-))
I was wondering if my France transformer my have died from this. However,
the levels it blew at, were far below where this would have been an issue...
Cheers,
Terry
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Date: Fri, 18 Feb 2000 20:07:41 -0700
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Subject: Matching capacitor size to transformers
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Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
Hi All,
I was pondering how to best predict what size capacitor would go with what
size NST. Also, given a NST and capacitor size, what would be the break rate.
I have come up with the following two equations based on many things:
0.92 x ( Vo x Io - L ) / ( BPS x Vo^2 ) = C
0.92 x ( Vo x Io - L ) / ( C x Vo^2 ) = BPS
Vo = Transformer RMS output voltage (volts)
Io = Transformer RMS output current (amps)
L = known system loss (mostly protection filter resistors) (watts)
BPS = Breaks Per Second
C = Capacitor value in Farads
The second equation is just a slight rearrangement of the first.
C is the largest cap size that a fine tuned LTR coil can charge. Static
gap and other systems would be less but at least this provides an upper
limit. The BPS equation my predict what the BPS of a static gap system
with say resonant charging might be.
This equation is meant to fill a void in my MMC program. The MMC program
is good at arranging small caps to make a given capacitor, but many people
don't know what value of capacitor they need...
The equation comes from how much energy a transformer can deliver and how
many times per second it can charge a cap of a given value. The 0.92
factor comes from my observations of my coils and basically is adjusting
for system loss.
Comments??
Cheers,
Terry
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